Generally there are four methods in use in the United States for manufacture of tablets, namely direct compression, dry powder blend, pre-compressed dry powder blend and wet granulation, as explained in U.S. Pat. No. 4,439,453.
In the direct compression method, all the required tabletting ingredients (active and aids) are incorporated into a free flowing granulation which is supplied to the manufacturer of bulk tablets. The granulation requires no pre-processing or blending with additional aids in order to be tabletted. Rather, the free flowing granulation supplied to the tablet manufacturer can be charged directly to a tabletting press.
The direct compression method is a generally preferred method for a number of reasons including economical reasons. The analgesic aspirin is generally tabletted using such a direct compression method since crystalline aspirin is soft and exhibits good plasticity/elasticity when compacted into tablets.
However, because the analgesic acetaminophen has significantly different properties than aspirin, it is generally considered to be non-compressible and not readily amendable to production of directly compressible granulations thereof. Generally, the less desirable wet granulation method of tabletting has been used to tablet acetaminophen. Generally, these wet granulation processes require large amounts of excipients, e.g., from about 25 to about 40% or more by weight of excipients. That is, in contrast to aspirin, the acetaminophen crystals are hard and brittle and are easily fractured. The acetaminophen crystals have essentially no plasticity/elasticity and, therefore, have required the use of unduly large amounts of aids, lubricants and/or excipients in order to be compressible into tablets by the direct compression method.
Therefore, there is a recognized need for a direct tabletting granulated acetaminophen composition that is free flowing and capable of being directly compressible into tablets. A further need is for such a directly compressible acetaminophen granulation composition to provide a high load, for example, at least 80%, or preferably at least 90% or more, of acetaminophen active in the composition. Thus, the amount of excipients required in the compositions should be kept quite low, for example, 20% or less, preferably 10% by weight or less. In addition, the directly compressible acetaminophen composition should readily be free flowing and readily permit dry blending with other active ingredients should that be desired or required. A further need is that the directly compressible acetaminophen composition be such as to provide good flow and compressibility characteristics so as to produce tablets of content uniformity, acceptable hardness and friability, and also provide a fluid bed granulation with a characteristic rough surface morphology and a high surface area suitable for good blending potential with other co-actives.